background extinctionsBackground extinctions are those extinctions that occur continually throughout time. These extinctions are caused by small changes in climate or habitat, depleted resources, competition, and other changes that require adaptation and flexibility. Most extinctions (perhaps up to 95 per cent of all extinctions) occur as background extinctions.

ballistic fragmentA ballistic fragment is a piece of rock that is ejected from a volcano with great speed.

basaltBasalt is a type of volcanic (igneous) rock. This hard, dark rock is composed of 45 to 54 percent silica (SiO2) and is often rich in iron and magnesium. Basalt is the most common type of rock in the Earth's crust - most of the sea floor is made up of basalt.

basaltic lavaBasaltic lava is a lava composed of molten basalt. This type of lava erupts at temperatures from 1100 to 1250° C. Basalt lava has a low viscosity (resistance to flow); it flows relatively quickly. Shield volcanoes are almost entirely composed of basalt.

base surgeA base surge is a cloud of rock debris, water, and steam that moves close to the ground at high speeds. It is formed by an explosive volcanic eruption.

batholithA batholith is a huge body of igneous rock that solidified under that Earth but now has at least 100 square kilometers (40 square miles) exposed. Half Dome at Yosemite National Park, California, USA, is a granite batholith.

bathypelagicBathypelagic means of, pertaining to, or living in the deep ocean near the bottom.

1,000,000,000

billionA billion is a thousand million. Multicellular life evolved on Earth about a billion years ago.

biomassBiomass is the amount of animal and plant life in an area. Biomass is usually expressed in units of weight per volume.

biotaBiota is the animal and plant life of an area.

blockA volcanic block is a chunk of rock (over 64 mm) that is ejected (thrown) from a volcano. Blocks usually originate from the volcano's cone and are solid chunks of older lava flows. A block is one size of tephra.

blowdownA blowdown is a area of trees that has been blown over by a volcanic blast.

bombA volcanic bomb is a chunk of viscous lava (over 64 mm) that is ejected (thrown) from a volcano. Since they are still viscous when ejected, they often have rounded, aerodynamic shapes. Types of bombs include: breadcrust bombs, ribbon bombs, spindle bombs, spheroidal bombs, and "cow-dung" bombs.